In the semiconductor industry, there is a continuing effort to increase device density by scaling device size. State of the art devices currently have device features with a dimension well below 1 micron (submicron). To form these features, a photosensitive layer is formed on a substrate or device layer, and is exposed to radiation through a reticle. The reticle typically comprises a substantially transparent base material with an opaque layer having the desired pattern formed thereon, as is well known. At the submicron level, diffraction effects become significant, resulting in exposure of portions of the photoresist layer underlying the opaque layer near the edges of features.
To minimize effects of diffraction, phase-shifted reticles have been used in the prior art. Typically, a phase-shifted reticle has an opening in the opaque layer corresponding to the pattern to be formed. Phase-shifters, which transmit the exposing radiation and shift the phase of the radiation approximately 180.degree. relative to the openings, lie along or near the outer edges of the features. The radiation transmitted through the phase-shifter destructively interferes with radiation from the opening, thereby reducing the intensity of radiation incident on the photoresist surface underlying the opaque layer near a feature edge.
Prior art phase-shifted reticles have a number of problems which limit their ability to be used to pattern some features, however. Often, it is desired to place two features in close proximity to one another. For example, contact or via openings may be placed in a closely spaced array. In prior art phase-shifted reticles, each opening has a phase-shifting rim surrounding it. In the case of contact openings in an array, the phase-shifting rims of two openings may be very close to one another. In this case, the phase-shifting rims of the two close opening patterns are roughly equivalent to one very wide rim. Unfortunately, as the phase-shifting rim width is increased, the intensity of radiation underneath the phase-shifting rims increases. The increased intensity causes a deep recession in the developed photoresist layer, and may in fact cause a portion of the photoresist to be removed between two openings. This proximity effect occurs if two phase-shifting rims are positioned at approximately 0.55 .lambda./NA or less, where K is the wavelength of the exposing radiation, and NA is the numerical aperture of the lens of the lithographic printer being used.
In addition to the above-described proximity effect, phase-shifted reticles may be difficult to produce for certain features. During the fabrication of the reticle, the phase-shifter may be fabricated using a separate masking step from its associated feature. For very small features, the alignment tolerance may be beyond the capability of the process. In addition, even if the phase-shifter can be produced, inspection and repair is often very difficult. Finally, physical limitations may prevent the use of phase-shifters for some types of features. Generally, the phase-shifter must have a width equal to approximately 0.1-4.4 IRF-.lambda./NA, where IRF is the image reduction factor of the lens. On a 1:1 basis, the width of the phase-shifters are typically approximately 0.1.mu.. Because phase-shifters have this finite width, not all device patterns can be fabricated with phase-shifted reticles, as portions of a feature or two individual features may be spaced too closely together to allow for placement of the phase-shifters in the appropriate regions.
To overcome these problems, an attenuated phase-shifted mask (APSM) has been proposed. The APSM replaces the opaque layer of prior art masks (which is typically a layer of chrome about 0.1.mu. thick) with a "leaky" layer which transmits a reduced or attenuated percentage of the radiation incident thereon. For example, a very thin layer of chrome (approximately 300 .ANG.) could be used as the leaky layer. A chrome layer this thin will transmit approximately 10% of the radiation incident on the reticle. Additionally, the leaky chrome layer shifts the phase of the transmitted radiation approximately 30.degree. compared to radiation transmitted through regions of the reticle where the leaky chrome layer is not present. In order to achieve the required 180.degree. shift, the features are phase-shifted an additional 150.degree., either by etching the reticle or by placing a phase-shifting material in the regions of the features. That is, the APSM comprises a layer of leaky chrome covering the entire reticle base, except for the features, which are open regions (i.e regions having no thin chrome layer) with appropriate phase-shifting. Since the radiation transmitted through the features is phase-shifted 180.degree. relative to radiation transmitted through the leaky chrome layer, the prior art phase-shifters are not required. The portion of the reticle outside of the features having the attenuating chrome layer will be referred to as the "field" of the reticle herein. Since the leaky chrome transmits only about 10% of the exposing radiation, regions away from the features are only partially exposed. This partial exposure removes some of the photoresist from these regions, but a continuous layer remains. The photoresist layer thickness is adjusted for this exposure. For example, if the photoresist layer underneath the leaky chrome is reduced in thickness 1,000 .ANG. due to the exposure, and a 10,000 .ANG. layer of photoresist after developing is desired, an initial photoresist layer of 11,0000 .ANG. will be used. The leaky chrome layer is also referred to as an "absorptive phase-shifter" and as a "halftone chrome layer."
Several problems occur in the manufacture feature of the prior art APSM. First, it is extremely difficult to deposit a thin layer of chrome with uniform thickness across the surface of the reticle. Typically, some regions will have a thickness greater than the target thickness and some regions will have a thickness less than the target thickness. Second, it is difficult to deposit this thin layer with uniform physical characteristics across the surface of the reticle. For example, some regions will have more residual gas (e.g., nitrogen) incorporated in the film than other regions. Because of this and other factors, physical properties such as the optional density and refractive index are not uniform. Finally, as is well know, the thin chrome layer will have a much higher pin hole density than the prior art opaque chrome layer. In addition to the above-described difficulties in depositing the thin chrome layer, it is difficult to maintain the quality of the deposited layer throughout the reticle manufacturing process. Since the layer is so thin, it is not as rigorous as the prior art opaque chrome layer. For example, during the various manufacturing steps, such as cleaning, the above-described non-uniformities and defects may become worse. These difficulties in producing and maintaining a high quality thin chrome layer limit the ability of the prior art APSM to achieve good lithographic performance, for the reasons described below.
One problem caused by the non-uniform thickness and refractive index of the thin chrome layer in the APSM is a varying "focal shift," which will be explained in reference to FIGS. 1A and 1B. In the following FIGS. 1A and 1B, a graphical representation of defocus versus critical dimension is shown. It will be understood that the actual values can vary considerably based upon the feature being formed, exposure parameters, including time and energy of the exposure, printer parameters and other factors. The FIGS. 1A and 1B provide an example for one set of exposure and printer parameters. Referring to FIG. 1A, a graph of critical dimension (CD) in a photoresist layer plotted against defocus (distance between photoresist layer and best or perfect focus) is shown. As can be seen from curve 10 of FIG. 1A, if the image is defocused in either the positive or negative direction, the dimension of an opening in the resist decreases. If the process specification allows for a CD in the range of 0.3-0.5 microns, then for the example shown in FIG. 1A, the defocus can be in the range of approximately -0.75.mu. through +0.75.mu., since the CD varies from about 0.3.mu. at -0.75 defocus, to 0.4.mu. at 0.mu. defocus, and back down to 0.3.mu. at +0.75.mu. defocus. Outside of this range, the CD falls below 0.3.mu. and is outside of the specified range. The range between -0.75.mu. and +0.75.mu. is the depth of field (DOF), and is shown by the line 11. Thus, so long as the photoresist layer is within this DOF, the CD will be within specification. As is well known, a large DOF is desirable, as the wafer topography and other factors cause the level of the photoresist layer to vary considerably across the exposure field of the printer.
In a phase-shifted reticle, the above-mentioned focal shift occurs, whereby curve 10 shifts as the phase difference between the phase-shifter and the feature varies from 180.degree. (phase error). If the phase difference is less than 180.degree., the curve 10 shifts to the right. If the phase difference is greater than 180.degree., the curve 10 shifts to the left. Referring to FIG. 1B, curve 12 shows a plot of CD versus defocus for a feature where the phase difference between the feature and its phase-shifter is less than 180.degree., and curve 13 shows CD versus defocus for a feature where the phase difference between the feature and its phase-shifter is greater than 180.degree.. Note that the shape of an individual curve does not change significantly due to phase error. Rather, the curve simply shifts, the direction and amount of shift determined by the direction and amount of phase error from 180.degree..
As mentioned earlier, the prior art thin chrome layer accounts for approximately 30.degree. of the phase-shift. However, this value will vary as the thickness and refractive index of the film varies. Since, as described above, the thickness and refractive index of the thin chrome layer are non-uniform, the phase-shift will be non-uniform across the surface of the reticle. Because of the varying phase-shift of the thin chrome layer, there will be regions on the prior art APSM where the phase difference is less than 180.degree. between a feature and its phase-shifter and regions where the phase difference is greater than 180.degree.. That is, the prior art APSM will typically have both negative and positive phase error. If the prior art APSM has some features where the phase difference is, for example, 170.degree., and some features where the phase difference is, for example, 190.degree., the DOF will be greatly decreased due to the focal shift. As can be seen from FIG. 1B, for those features having a phase difference less than 180.degree. (curve 12), the CD will be below 0.3.mu. if the defocus is approximately 0.5.mu. or greater, while those features having a phase difference greater than 180.degree. (curve 13) will have a CD below 0.3.mu. if the defocus is approximately -0.5.mu. or less. Thus, DOF 14 now extends only from -0.51.mu. through +0.5.mu. due to the varying phase error. This compares to DOF 11 of -0.75.mu. through +0.75.mu. for a reticle with no phase error, or uniform phase error.
A problem caused by the non-uniform thickness and optical density of the thin chrome layer in the prior art APSM is that light transmission through the layer will not be uniform. This may cause excessive removal of photoresist in some areas. Additionally, if the transmittance near features is too low, there may be insufficient phase-shifted radiation to effectively minimize diffraction, thereby causing a loss in resolution.
Finally, the increased pin hole density of the prior art APSM will cause undesired openings to be created in a photoresist layer exposed by the APSM. If the openings occur in critical regions, the APSM will be unusable.
What is needed is an attenuated phase-shifted reticle which allows for use of phase-shifting to improve resolution, which can be used on all features and closely spaced features, which has only minor or relatively uniform phase error, which has a low defect density, and which is easily manufacturable using current technology.